What is the stroma in chloroplasts primarily responsible for?

The stroma in chloroplasts is primarily responsible for serving as the site for the Calvin cycle. This process is essential for photosynthesis, as it occurs in the stroma and utilizes carbon dioxide to synthesize glucose, using the energy derived from ATP and NADPH produced in the light-dependent reactions.

During the Calvin cycle, key enzymes and substrates are present in the stroma, allowing for the fixation of carbon dioxide and the eventual production of organic molecules. This makes the stroma vital for converting solar energy into chemical energy stored in carbohydrates, which are crucial for plant metabolism and growth.

The other options highlight functions that are important in photosynthesis but do not pertain directly to the role of the stroma. Sunlight absorption is primarily conducted by chlorophyll located within the thylakoid membranes, and gas exchange occurs through stomata, not in the stroma itself. While chlorophyll is housed in the thylakoids, it does not describe the main function of the stroma where the Calvin cycle takes place.